A. Field of Invention
The present invention provides novel hydroboration agents and more particularly relates to a novel class of borane adducts with N,N-dialkylanilines.
B. Prior Art
Borane adducts with amines are versatile reagents exhibiting many different properties as compared to the metal borohydrides. For example, they are soluble in a variety of solvents, including hydrocarbons or even water, and in some cases can be used in an acidic medium. Many adducts have been synthesized. See for example, Long, L. H. in W. J. Mellor A Comprehensive Treatise on Inorganic and Theoretical Chemistry; Longman: London, 1981, Supplement Vol. 5, Part B1, p 1.; and Meller, A. In Gmelin Handbook of Inorganic and Organometallic Chemistry; Springer: Berlin, 1992, 4th Supplement, Vol. 3, p 1. Several are commercially available. They find various uses, e.g., as fuel additives, polymerization catalysts, polymer stabilizers and stain removers, in metal plating and in the dye and pharmaceutical industries. See Lane, C. F. Aldrichimica Acta 1973, 6, 51. Most of these applications are based on their reducing properties.
In contrast, the use of borane-amine adducts for hydroboration is rather limited due to strong complexation, which renders their reactivity low as compared to the weak borane adducts with ethers and sulfides. For example, borane-triethylamine does not hydroborate 1-octene at room temperature and only very slowly in refluxing tetrahydrofuran (THF). See Brown, H. C. et al. Inorganic Chem. 1984, 23, 2746.
Borane adducts with the less basic anilines are weaker and hence more reactive. See Brown, H. C. et al. Inorganic Chem. 1984, 23, 2746; Narayana, C. et al. Organometal. Chem. 1987, 23, 145; and Camacho, C. et al. Synthesis 1982, 1027. Borane-N,N-diethylaniline hydroborates 1-octene in tetrahydrofuran at room temperature in 2 hours. The adducts with N-phenylmorpholine and N-phenylaniline are still more reactive. However, they are air-sensitive solids which are considered less convenient to handle than liquids for large-scale commercial applications.
Borane-tetrahydrofuran is a valuable reagent for the hydroboration of olefins and for the reduction of organic compounds. It suffers from the disadvantage in that the solutions are unstable over a period of time. U.S. Pat. No. 3,882,037 discloses stabilized borane-tetrahydrofuran solutions which permit storage of such solutions for relatively longer periods of time. However, the inherent availability only as a relatively dilute solution in tetrahydrofuran poses a drawback to commercial use of this reagent.
Borane-methyl sulfide (BMS) is much more stable than borane-tetrahydrofuran and is widely used for both hydroboration and reduction [See Burg et al., J. Am. Chem. Sec. 76, 3307 (1954) and Coyle et al., J. Am. Chem. Sec. 81, 2989 (1959)]. However, it suffers from the serious disadvantage in that it yields a product which contains free dimethyl sulfide. The free dimethyl sulfide is highly volatile, b.p. 38.degree. C., flammable and has a very noxious odor. Moreover, it is not soluble in water, so it cannot be disposed of by washing it away with water.
Borane-1,4-thioxane (U.S. Pat. No. 4,298,750) is another valuable hydroboration agent. It has both lower volatility and milder odor than dimethyl sulfide. It has a limited solubility in water and can be easily oxidized to the corresponding sulfoxide, which is miscible in water. This agent is a liquid, 8M in BH.sub.3, stable over prolonged periods. Unfortunately, this commercially available reagent is relatively costly compared to borane-tetrahydrofuran and borane-dimethyl sulfide.
The growing importance of borane reagents for the synthesis of pharmaceuticals and other compounds and the problems associated with other well established borane adduct hydroboration agents, e.g., low concentration and stability, high volatility, flammability, unpleasant odor, as discussed above, create a need for easy to handle, stable and environmentally benign hydroborating agents as discussed specifically below. The present invention fulfills that need.